The present invention relates to a developing device for transferring developing agents to an image carrier in an image forming apparatus.
In an image forming apparatus, such as an electronic copying machine, a developing device is customarily used for developing an electrostatic image formed on an image carrier such as a photosensitive body. One-component developers or two-component developers are used in developing apparatus of this type. Two-component developers include toner particles contributing to development and carrier particles for properly charging the toner particles. One-component developers are classified into magnetic and nonmagnetic developers.
A developing device using a non-magnetic single-component developing agent is disclosed in U.S. Pat. No. 4,745,429 by Mukai et al. This developing device includes a developing roller facing a photosensitive body, a toner supply roller which supplies toner to the developing roller and a coating blade which is pressed against the developing roller. Rotation of the toner supply roller causes toner particles to be transferred to the developing roller so as to form a thin toner layer on the developing roller. As the developing roller is rotated, a single-component toner, as a single-component developing agent, adheres to the surface of the developing roller by triboelectronic charges between a stainless steel coating blade and single-component toner particles, and between the developing roller and the single-component toner particles. Then, the single-component toner particles, supported on the developing roller are caused to adhere electrostatically to an electric image formed on a photosensitive body at the developing station facing the photosensitive body.
In the device described above, after an unusually large amount of toner is consumed for developing, such as for developing a solid toner image, the amount of toner transferred from the toner supply roller to the developing roller increases, but the amount of toner supported on the toner supply roller decreases. This is because insufficient time exists for friction between the toner supply roller, developing roller and toner to increase the charge on the toner sufficiently for enough toner to adhere to the toner supply roller. If only the rotation of the toner supply roller is relied upon to charge the toner, it is impossible to supply a sufficient amount of toner to achieve developing with a high image density.
A developing device using a magnetic single-component developing agent or a two-component developing agent is disclosed in U.S. Pat. No. 4,244,322. A developing roller includes a inside of the sleeve. The developing agent is magnetically supported on the surface of the sleeve so as to form a magnetic brush. The magnetic brush contacts the photosensitive body to develop the electrostatic latent image. In the device described above, after an unusually large amount of toner is consumed for developing, the magnetic brush is not sufficiently formed. Therefore, the density of the toner image becomes low.
In both devices as described above, the amount of toner supported on the developing roller is limited. For this reason, directly after an unusually large amount of toner is consumed, for example, for a solid toner region, it is impossible to supply the necessary amount of toner to the developing roller. For this reason, there is a problem that the density of the toner image becomes very low.